Computed tomography (CT) is known as a two-stage imaging method. In this method projection data is recorded by an examination object being irradiated with x-ray radiation and the attenuation of the x-rays on their way from the x-ray source to the x-ray detector being recorded. The attenuation is caused by the irradiated materials along the beam path, so that the attenuation can also be understood as a line integral over the attenuation coefficients of all volume elements (voxels) along the beam path. The recorded projection data is not able to be interpreted directly, i.e. it does not produce an image of the irradiated slice of the examination object. Only in a second step is it possible, using reconstruction methods, to compute back from the projection data to the attenuation coefficients of the individual voxels and thus create a picture of the distribution of the attenuation coefficients. In such cases the projection data can comprise a plurality of individual projections which have been recorded from different projection angles. The recording of a plurality of individual projections from different projection angles is also referred to as a scan.
Image data, which represents a two-dimensional image in the form of a slice of the examination object for example, can be reconstructed on the basis of the projection data created during the scan. For example image data also comprises a plurality of two-dimensional images or a contiguous three-dimensional volume. Problems during the reconstruction of the image data emerge if, during the recording of the projection data described above, the examination object protrudes for at least a few of the projection angles beyond the scanning area of the system consisting of x-ray source and x-ray detector. The scanning area is defined by the relative spatial arrangement of the x-rays generated by the x-ray source and the x-ray detector. Within the scanning area complete projection data can be obtained; outside the scanning area complete projection data cannot be obtained. Thus if the examination object protrudes beyond the scanning area, projection data recorded during the irradiation of the examination object is incomplete, which leads to image artifacts during the reconstruction. In order, despite this, to still make possible an image reconstruction which is as precise as possible, a corresponding expansion of the projection data is required for the incomplete projections before the reconstruction. The scanning area is also referred to as the field of view, and methods for reconstruction of image data from the incomplete projection data are also referred to as methods for reconstruction in the expanded field of view. The term scanning area and field of view are used synonymously below.
A method for reconstruction of image data from incomplete measurement data is known from DE 102010006585 A1. In this method first image data is reconstructed from the measurement data and on the basis of the first image data a delimitation of the examination object is determined. Subsequently the first image data is modified using the determined delimitation and projection data is calculated from the modified first image data. The measurement data is modified using the projection data and finally second image data is reconstructed from the modified measurement data.